Nitric oxide (NO) is the first gaseous molecule that acts as a biological messenger and participates in a myriad of biological processes including control of blood pressure, neurotransmission and inhibition of tumor growth. The desire to deliver NO at biological targets under specific physiological conditions has inspired research in the area of designed molecules that release NO on demand. Although a few organic exogenous NO donors (like nitrates and nitrosothiols) and sodium nitroprusside (NSP) have been utilized for such purpose, the possibility of non-porphyrin metal nitrosyls as NO donors has not been explored in any systematic way.
The discovery of the roles of nitric oxide (NO) in blood pressure control, neurotransmission, and immune response has stimulated extensive research activity in the chemistry, biology, and pharmacology of NO.1-5 Cellular NO is almost exclusively generated via oxidation of L-arginine (reaction 1) by the enzyme nitric oxide synthase (NOS).6 NO synthases are a class of heme-proteins which are both constitutive (e.g., neuronal and endothelial NOS) and inducible (e.g., cytokine-inducible NOS III). The principal targets of NO in bioregulatory processes are also iron proteins. For example, binding of NO to the Fe(II) center of the ferroheme enzyme soluble guanylyl cyclase (sGC) activates the enzyme leading to formation of the secondary messenger cyclic-guanylyl monophosphate (from guanylyl triphosphate), which in turn causes relaxation of smooth muscle tissue of blood vessels. Although NO concentrations of less than 1 mM are involved in endothelium cells for blood pressure control, NO concentrations produced during immune response to pathogen invasion are much higher. Sudden increases in local NO concentration have been exploited in NO-mediated photoinduced death of human malignant cells.7 
The desire to deliver NO at biological targets under physiological conditions has inspired research in the area of designed molecules that release NO on demand. Several reviews published in the past few years attest the extent of attention and interest in controlled NO release under specific conditions and development of NO-donor compounds.8 Currently, the major classes of exogenous NO donors include organic nitrates (e.g., glyceryl trinitrate, GTN), nitrites (e.g., isoamyl nitrite, IAMN), nitrosamines, oximes, hydroxylamines, nitrosothiols (e.g., SNAP and GSNO), heterocycles (e.g., SIN-1) and NONOates (e.g., DEA-NO).8 These compounds (FIG. 1) release NO upon exposure to heat, light, oxidants or thiols, and in some cases via enzymatic reactions. Several of these organic compounds have found use as pharmaceuticals. In addition, metal-NO complexes (nitrosyls) like sodium nitroprusside (SNP) and Roussin's salts have also been exploited as NO donors (FIG. 2).2,8 
There are however, fundamental problems associated with conventional NO-donors such as GTN, SNAP, IAMN, DEA-NO, as well as the metal-NO complex SNP, when used in biomedical applications. In particular, NO-release is poorly controlled upon administration of these NO-donors to a subject, since NO is released via various enzymatic and non-enzymatic pathways. For example, organic nitrates like GTN have been used to relieve angina pectoris and acute myocardial infraction, while organic nitrites such as isobutyl and isoamyl nitrite have been used clinically as vasodilators. The requirement for specific thiols and/or enzymatic bioactivation for triggering NO release from these drugs renders them less ideal compounds for the generation of predictable rates of NO release. Patients also develop nitrate tolerance in many cases. Nitrosothiols NO-donors such as SNAP and related derivatives are often unstable and production of NO is induced by heat, UV light, certain metal ions, superoxide and selected enzymes. In addition, NO-donors such as like hydroxyureas, hydroxylamines, and SIN-1 (sydnonimines) all require activation by specific enzymes and are difficult to target to a given site.
Production of secondary toxic product(s) is another problem encountered with certain conventional NO-donors. For example, among the inorganic NO-donors, sodium nitroprusside (SNP, marketed as NIPRIDE or NITROPRESS) has been in therapeutic use for quite some time. SNP is widely used in hospitals to lower blood pressure during hypertensive episodes (e.g., heart attacks and congestive heart failures). It is also used to combat vasoconstriction during open-heart surgery. However, cyanide poisoning poses some risk in SNP therapy, and in fact SNP infusion is often discontinued after 10-15 min to minimize cyanide toxicity. Moreover, deaths of several patients that have received SNP therapy have been reported (e.g., Butler and Megson, Chem Rev, 102:1155-1165, 2002). The cyanide ions, released during photolysis or in vivo enzymatic reduction of SNP are metabolized in the liver and kidneys by the enzyme rhodanase, which converts CN− to SCN−. Patients with severe hepatic compromise therefore require strict monitoring of the thiocyanate levels and are often not good candidates for SNP therapy. Synthetic iron-sulfur cluster nitrosyls like Roussin's black salt (RBS, [Fe4S3(NO)7]−), Roussin's red salt (RRS, [Fe2S2(NO)4]2−) and [FeNOS]4 (FIG. 2) have also been employed therapeutically. Although photodecomposition of RRS and RBS generates NO, formation of ferric precipitates often limits their use. In addition, RRS exhibits carcinogenic properties.
Thus, what is needed in the art are compositions and methods comprising NO-donors that can be safely administered to subjects (e.g., patients with heart disease or caner), and whose release of NO can be satisfactorily regulated. The present invention meets this need by providing compositions and methods comprising NO-complexes that release NO upon illumination with low-power light.